I would like to change the commentarystyle to styleD designed as following in the .bbx file:

\newbibmacro{customa:styleD}{% <-- This is actually a test, not having any idea
\bibsstring{\thefield{series}}% how it will look like, being unable to make it appear
\printtext{-}% on the pdf result
\printnames{labelname}%
\addcomma\addspace%
\printfield{titleaddon}\addspace%
\printfield{part}%
}%

And would supposably look like the:

BsK-Huguenin/Meize, OR 19/20 N 12.

Styles A, B and C look like this (these are somewhat modified compared to the original package, most of it concerning the punctuation without the use of \newblock nor \newunit):

Without the last ligne, I can't load the pkg with the styleD option, but I can't figure out what the ending (coming after the last \, like z@, @ne, tw@ for styles A, B and C respectively), since if I copy one of the other styles, the said style is used instead of the styleD.

Anyone has an idea on what combination of letters and @ to add at the end of the last line to make it possible to load?

Cheers,

Archein

The additional infos

P.S.: if you ever go looking at the original biblatex-swiss-legal package, it's actually quite old (Jan 2014) and doesn't take into consideration the new biblatex/biber changes since then, I already corrected the file.

P.S.S.: I beg for mercy if this question might seem stupid or easy to solve, I still new and trying to get my hands on biblatex.

Do you want to be able to use the commentarystyle option to choose your style? If not you could just do \newbibmacro{customa:styleA} and overwrite the A style (analogous for B or C). The code to switch between the style looks mighty complicated right now, but that could also be tackled if you insist.
– moeweAug 28 '16 at 16:38

(BTW: Please not that your MWE is not actuallyan MWE. This question probably doesn't need one - and it would not be easy to create one anyway since the biblatex style will probably throw all sorts of errors with a recent biblatex version. But for future reference, here's what makes a good MWE).
– moeweAug 28 '16 at 16:41

@moewe: no no, I would like to maintain styles A, B and C if possible, while still creating a new D style of my own for special kinds of documents.
– Archein von DrakenovAug 28 '16 at 16:50

@moewe This is going to be pretty hard to answer with a more complete example, though. If we don't know what changes have been made to the style, it is hard to be sure what will not conflict. Moreover, it is impossible to test any solution without updating that file to make it work with current packages. Since the OP has already done that, a link to the updated version (probably would need to be renamed for licence reasons) would surely make it easier. That's not an MWE, but an MWE wouldn't do any harm for testing, either.
– cfrAug 28 '16 at 23:50